The present invention relates to a thin film transistor and a method for fabricating the thin film transistor, more specifically to a highly reliable thin film transistor using low resistance wiring, and a method for fabricating the thin film transistor.
Liquid crystal display devices have an advantage that they are thin and light, and can be operated at low voltages with small current consumption. Recently liquid crystal display devices are widely used as displays of personal computers, etc.
Generally, the display panels of liquid crystal display devices are each constituted with two transparent glass substrates and liquid crystal sealed between the two transparent glass substrates. On one of the opposed sides of the two glass substrates a black matrix, a color filter, an opposed electrode, an alignment film, etc. are formed, and thin film transistors, picture element electrodes and an alignment film are formed on the other of the opposed sides of the two glass substrates.
Polarization plates are adhered respectively to the sides of the two glass substrates, which are opposite to the opposed sides. The polarization axes of the two polarization plates are arranged normal to each other to provide a liquid crystal display of normally white mode. That is, light is transmitted when no electric filed is applied to the liquid crystal, and when an electric field is applied to the liquid crystal, light is shaded. On the other hand, the polarization axes of the two polarization plates are parallel with each other to provide the liquid crystal device of normally black mode. That is, light is shaded with no electric field applied to the liquid crystal, and light is transmitted with an electric field applied to the liquid crystal.
A conventional liquid crystal display device will be explained with reference to FIGS. 11A and 11B. FIG. 11A is a plan view of a conventional active matrix substrate of the invert stagger type. FIG. 11B is a sectional view of the active matrix substrate along the line A-Axe2x80x2 in FIG. 11A.
As shown in FIG. 11B, a gate electrode 118 is formed of an Al film 112 and an Mo film 116 on a glass substrate 110. As shown in FIG. 11A, the gate electrode 118 is connected to a gate bus line 118a of the same conductor films.
Al film 112 is used as a material of the gate electrode 118 because Al has low electric resistance. In the conventional liquid crystal devices Cr, etc., which are metals of relatively high electric resistance and high melting point, have been used. Recently, in accordance with large scales and higher definition of the liquid crystal display devices, low resistance materials, such as Al, etc., are used.
The Mo film 116 is formed on the Al film 112 because Mo has high heat resistance and makes good electric contact with the Al film 112 with the other wiring, etc. The gate bus line 118a is connected to TAB through ITO (Indium Tin Oxide) in a region not shown, but is connected to other wiring, etc. through the Mo film 116. The gate bus line 118a can have good electric contact.
A gate insulation film 120 is formed on the glass substrate 110 with the gate electrode 118 formed on. An amorphous silicon film 122 is formed on the gate insulation film 120. A channel protection film 124 is formed on the amorphous silicon film 122. An n+-amorphous silicon film 126 is formed on the amorphous silicon film 122 with the channel protection film 124 formed on. A source electrode 136a and a drain electrode 136b are formed of an Mo film 128, an Al film 130 and an Mo film 134 on the n+-amorphous silicon film 126. As shown in FIG. 11A, the drain electrode 136b functions as a data bus line.
A protection film 138 is formed on the gate insulation film 120 with the source electrode 136a and the drain electrode 136b formed on. A contact hole 140 arriving at the source electrode 136a is formed in the protection film 138. A picture element electrode 142 is formed of ITO on the protection film 138 connected to the source electrode 136a through the contact hole 140. The Al film 130 is connected to the picture element electrode 142 through the Mo film 134, and the electric contact is good.
As described above, in the conventional liquid crystal display device shown in FIGS. 11A and 11B, Al, which is a low resistance metal, is used as a material of the gate bus line and the data bus line, and is suitable for larger scales and higher definition.
However, in the liquid crystal display device shown in FIGS. 11A and 11B, the side surfaces of the Mo film 116 of the gate electrode 118 is acute, which makes the step coverage of the gate insulation film 120 poor. Film quality of the gate insulation film 120 is interrupted near the side surfaces of the Mo film 116. Accordingly, the gate insulation film 120 has low dielectric voltage resistance.
The side surfaces of the Mo film 134 of the source-drain electrodes 136a, 136b are acute, which makes it difficult to form the protection film 138 in good quality. The protection film 128 has low dielectric voltage resistance.
An object of the present invention is to provide a thin film transistor which uses a low resistance metal as a material of the gate electrodes and wiring but can ensure high reliability, and a method for fabricating the thin film transistor.
The above-described object is attained by a thin film transistor comprising a gate electrode formed on a substrate, a gate insulation film formed on the gate electrode, a semiconductor layer formed on the gate insulation film, and a source electrode and a drain electrode formed on the semiconductor layer, the gate electrode, the source electrode or the drain electrode including a first conductor film, a second conductor film formed on the first conductor film, and a third conductor film formed on the second conductor film; the first conductor film being formed of a metal selected out of Al, Cu and Ag, or an alloy of a metal, as a main component, selected out of Al, Cu and Ag, and having side surfaces sloped; the second conductor film being formed of a film of Mo containing nitrogen, or an alloy of Mo containing nitrogen, and having side surfaces sloped; and the third conductor film being formed of Mo, or an alloy of Mo as a main component. The gate insulation film is formed on the gate electrode having the side surfaces generally sloped, whereby film quality of the gate insulation film is prevented from being interrupted near the side surfaces of the gate electrode. The gate insulation film can be highly reliable and can have high dielectric voltage resistance. The thin film transistor can be highly reliable.
The above-described object is attained by a thin film transistor comprising a gate electrode formed on a substrate, a gate insulation film formed on the gate electrode, a semiconductor layer formed on the gate insulation film, and a source electrode and a drain electrode formed on the semiconductor layer, the gate electrode, the source electrode or the drain electrode including a first conductor film, and a second conductor film formed on the first conductor film; the first conductor film being formed of a metal selected out of Al, Cu and Ag, or an alloy of a metal, as a main component, selected out of Al, Cu and Ag, and having side surfaces sloped; the second conductor film including a lower layer formed of a film of Mo containing nitrogen or an alloy of Mo, as a main component, containing nitrogen, and an upper layer formed of a film of Mo or an alloy of Mo, as a main component, and side surfaces of the lower layer being sloped. The gate insulation film is formed on the gate electrode having the side surfaces generally sloped, whereby film quality of the gate insulation film is prevented from being interrupted near the side surfaces of the gate electrode. The gate insulation film can be highly reliable and can have high dielectric voltage resistance. The thin film transistor can be highly reliable.
The above-described object is attained by a method for fabricating a thin film transistor comprising the steps of forming a gate electrode on a substrate, forming an gate insulation film on the gate electrode, forming a semiconductor layer on the gate insulation film, and forming a source electrode and a drain electrode on the semiconductor layer, the step of forming the gate electrode, or the step of forming the source electrode and a drain electrode including the steps of: forming a first conductor film of a metal selected out of Al, Cu and Ag, or an alloy of a metal, as a main component, selected out of Al, Cu and Ag; forming a second conductor film of Mo containing nitrogen or an alloy of Mo, as a main component, containing nitrogen; forming a third conductor film of Mo or an alloy of Mo as a main component; and etching the second conductor film at a higher etching rate than the first conductor film, and etching the third conductor film at a higher etching rate than the second conductor film to thereby slope side surfaces of the first conductor film and of the second conductor film. The gate insulation film is formed on the gate electrode having the side surfaces generally sloped, whereby film quality of the gate insulation film is prevented from being interrupted near the side surfaces of the gate electrode. The gate insulation film can be highly reliable and can have high dielectric voltage resistance. The thin film transistor can be highly reliable.
The above-described object is attained by a method for fabricating a thin film transistor comprising the steps of forming a gate electrode on a substrate, forming an gate insulation film on the gate electrode, forming a semiconductor layer on the gate insulation film, and forming a source electrode and a drain electrode on the semiconductor layer, the step of forming the gate electrode, or the step of forming the source electrode and the drain electrode including the steps of: forming a first conductor film of a metal selected out of Al, Cu and Ag, or an alloy of a metal, as a main component, selected out of Al, Cu and Ag; forming a second conductor film including a lower layer of Mo containing nitrogen or an alloy of Mo, as a main component, containing nitrogen, and an upper layer of Mo or an alloy of Mo as a main component; and etching the lower layer of the second conductor film at a higher etching rate than the first conductor film, and etching the upper layer of the second conductor film at a higher etching rate than the lower layer of the second conductor film to thereby slope side surfaces of the first conductor film and side surfaces of the lower layer of the second conductor film. The gate insulation film is formed on the gate electrode having the side surfaces generally sloped, whereby film quality of the gate insulation film is prevented from being interrupted near the side surfaces of the gate electrode. The gate insulation film can be highly reliable and can have high dielectric voltage resistance. The thin film transistor can be highly reliable.
As described above, according to the present invention, the gate electrode is formed of an AlNd film, an Mo film containing nitrogen, which can be etched at a higher etching rate than the AlNd film, and an Mo film, which can be etched at a higher etching rate than the Mo film containing nitrogen, whereby the gate electrode can be formed with the side surfaces generally sloped. The gate insulation film, which is formed on such gate electrode, can be kept from interruption of film quality near the side surfaces of the gate electrode. According to the present invention, the gate insulation film can have high reliability and high dielectric voltage resistance. Accordingly, the thin film transistor can have high reliability.
According to the present invention, the same technique that is applied to the gate electrode is applied also to the source/drain electrodes, whereby the source/drain electrodes can be formed with the side surfaces generally sloped. The protection film is formed on such source/drain electrodes, whereby the protection film is kept from interruption of film quality near the side surfaces of the source/drain electrodes. Thus, according to the present invention, the protection film can have higher dielectric voltage resistance, which leads to higher reliability of liquid crystal display devices.